Bending tests are known from the prior art as standard methods for characterising mechanical material properties. In the case of bending tests, a bending sample is usually arranged and optionally clamped on a mount. Subsequently, the bending sample is subjected to a mechanical load, for example to a continuously increasing force along a determined direction or to a force alternating in direction. By measuring a deformation caused by the force, in particular a bending angle or also a break angle, mechanical characteristic values of the material of the sample can be directly measured or calculated.
Useful variants of the bending test are the so-called 3 point bending test, in particular as a platelet bending test or the 4 point bending test. The bending sample is applied on two supports (the first two points). Using a bending punch or bending rail, a force is exerted between the supports on the bending sample, either using a bending punch or bending rail with a contact point (in the 3 point bending test) or using a bending punch or bending rail with two contact points (in the 4 point bending test). The sample between the supports is deformed by the force exerted by the bending punch or bending rail, for example in the 3 point bending test substantially in a V-shape with a determined opening angle or bending angle. For example, a characteristic curve is thereby recorded, in the case of which the force over the punch movement is measured and evaluated.
In such bending tests, the measurement result is dependent on the distance of the supports. What is problematic in this case is that the forces exerted on the supports increase very strongly with increasing bending angle such that the distance of the supports during a bending test can change with the bending angle through the yielding of the measuring device and thus the measurement values are distorted. The distance of the counter bearings should be kept constant as far as possible even under high forces in order to obtain more precise measurement results.
At the same time, it is, however, often desirable for the distance of the supports to be adjustable. In this case, different distances of the supports are in particular used for different sample geometries and/or bending punch geometries. Different distances can also be used within a series of measurements for a sample geometry.
DE 31 01 422 A1 describes a device for performing a bending rest having supports located on bearing blocks, wherein the bearing blocks are arranged adjustably on a base plate. The bearing blocks are thereby clasped in a groove on the base plate and displaceable towards each other via a thread shaft. The distance of the supports can thus be set by positioning the bearing blocks.
What is disadvantageous here however, is, on the one hand that the fixation of the bearing blocks against each other via the thread shaft may initially be subject to a certain play, which a thread connection involves. On the other hand, in the case of a bending test the thread shaft is also loaded via a bending force with height of the bearing blocks as a lever, which, in the case of larger forces results in a change of the distance of the supports. Hitherto, a compromise between precise adjustability of the distance of the supports and a high force resistance of the distance of the supports or a high rigidity of the measuring device had to be made.